The first thing to be made is to activate Upgrade option from "Software Updater".

Go To Software Updater>Settings and change the last dropdown (Notify me of new Ubuntu) from "Never" or "for long term support version" to "For any new version"
Close "Software Updater" and do a reboot/restart.

After restart you'll see that Upgrade button, is there on "Software Updater". Unfortunately is not possible to migrate directly to "Ubuntu 15.04 Vivid Vervet", first step is to migrate to "Ubuntu 14.10 Utopic Unicorn", and then, step 2 is to migrate from "Ubuntu 14.10 Utopic Unicorn" to "Ubuntu 15.04 Vivid Vervet".

On SSD drive can take 2 hours for these to migrations. I didn't noticed major problems with softwares after Upgrade, except VMPlayer which refuse to work :)

Special note for MacBookPro users:

After "Ubuntu 14.10 Utopic Unicorn" was installed, I lose completely the grub from Ubuntu. I was not able to see Ubuntu in boot options till I installed rEFIt. Was really annoying, without Ubuntu Live CD or Ubuntu USB installer to get back the boot options. I didn't noticed such thing when upgrade is made on dual OS machine having Windows and Ubuntu. Is just happening on MacBookPro with Mac OS X and Ubuntu.

# After reboot you'll see the options again,
# can be that are no icons or text,
# just click on the first item ;)

Emergency boot mode :)

For different reasons it may be that Ubuntu installation didn't work very well, you lose the internet/network connection during installation, installer just stop because of some error. This means that you're not gona get the vivid working, not yet.

Here how to fix this problem. Start again the computer, chose "Recovery Mode from GRUB Boot", you'll find this on "Advanced options for Ubuntu". Chose one of recovery modes from list.